The Heating Ventilating and Air Conditioning (HVAC) systems in use today use significant amounts of energy in accomplishing their designated task. Thus, it is desirable to make them operate efficiently and reduce the energy usage where ever possible. One drawback to most typical systems is that for cost and practical complexity reasons, they are designed to run at a fixed nominal operating point of maximum design efficiency. However, in typical use, the operating conditions span a large range above and below the nominal design point. When the typical HVAC system is running in the ranges above or below its design operating point, the system design is compromised. Worst case extremes are typically known so the system is designed to operate without sustaining damage, although not necessarily with optimum efficiency, at these extremes.
FIG. 1 illustrates a typical residential forced air HVAC split system comprised of an indoor unit 2 connected via refrigerant lines 4 to an outdoor unit 6. When the temperature inside the residence exceeds the set point of thermostat 8, the indoor unit 2 and the outdoor unit 6 are activated via signals in control lines 10 causing conditioned supply air to flow through duct 12 and be circulated throughout the residence. When the temperature reaches the set point of thermostat 8 the signals in the control lines 10 change and the indoor unit 2 and the outdoor unit 6 are switched off. The equipment remains at rest until the set point of thermostat 8 is exceeded once again.
FIG. 2 is a block diagram of a prior art HVAC refrigeration system suitable for use in the forced air HVAC split system of FIG. 1. Briefly described, a refrigerant gas is compressed by a compressor 20 and flows through line 21 to a condenser 22 where it is cooled and condensed to liquid by a heat exchange media circulator 24. The pressurized liquid refrigerant flows through line 26 to an evaporator 28 where it is heated and evaporated to a gas by a heat exchange media circulator 30. The resulting low pressure gaseous refrigerant flows through line 32 from the evaporator 28 back to the compressor 20 completing the cycle. Typically the compressor 20 will run at a constant speed, and the ability of the HVAC system to adapt to changes in the applied refrigeration load, such as seasonal changes, is limited. For example, in many cases, the heat exchange media flowing through the evaporator 28 can be moisture laden air or circulating water. Care must be taken to insure that a practical HVAC refrigeration system does not allow the evaporator temperature to fall below the freezing point of water. When moisture laden air contacts the evaporator 28 and the temperature of the evaporator 28 is below the dew point of the moisture laden air, the moisture in the air will condense on the surface of the evaporator 28. If the temperature of the evaporator 28 were allowed to fall below the freezing point of water, the moisture in the air passing through the evaporator would not only condense to a liquid state (water) but would also continue to cool further to freeze to a solid state (ice). If the heat exchange surface of the evaporator 28 becomes coated with ice, initially efficiency is reduced and ultimately all flow of air blocked. Likewise, in the case where the heat exchange media through the evaporator 28 is circulating water, the temperature of the evaporator 28 must be maintained above the freezing point of water to prevent ice from building up and blocking the flow of the circulating water.
A variety of regulating valves and variable orifices have been employed in conventional systems to control the expansion of the refrigerant in the evaporator and regulate the temperature in the evaporator to prevent such freezing conditions. They operate by reducing the flow of refrigerant through the evaporator. Unfortunately, reducing the flow through the evaporator may cause excessive pressure to build up in the condenser as the compressor continues to run. When condenser pressure increases, more energy may be unnecessarily consumed by the compressor. Many residential HVAC systems employ capillary tubing to control the expansion in the evaporator. While generally cost effective and reliable, capillary tubing function is fixed and cannot adjust to control the temperature in the evaporator. These systems with capillary tubing rely on a critical ideal amount of refrigerant charge in the system to achieve optimal performance. Because the refrigerant charge is fixed, this optimal performance is only achieved at one operating point.
Further problems are caused because the fixed refrigeration charge is typically added to the HVAC system during field installation. Often in the field, the lengths of refrigeration tubing between the indoor and outdoor units are not exactly known and thus the required ideal refrigerant charge is approximated by the tradesman preforming the installation. The resulting installed HVAC system typically operates in a compromised mode over varying load conditions such as presented by seasonal and daily weather changes. Occasionally it may operate at its ideal efficiency when the load conditions happen to coincide with the load conditions that correspond to the ideal load for the actual installed refrigerant charge. Additionally, although designed to be completely sealed, refrigerant can leak from sealed systems by escaping through fine cracks in fittings and welded connections, porous sections of castings, compressor shaft seals, and testing and servicing ports. The slow but continuous migration of refrigerant out of an operating refrigeration system may cause the ideal operating point of that system to change as the total refrigerant charge is reduced. Thus, an unresolved need continues to exist in the industry for systems and methods that enable more efficient operation of HVAC systems over a wider range of environmental operating conditions.
In a conventional system, such as illustrated in FIGS. 1 and 2, the fixed capacity of the HVAC system components are typically estimated according to the size and thermal loading of the residence at the maximum or worst case condition. However, in actual use, the operating conditions vary widely as the outdoor temperature and humidity varies on a day-by-day and hour-by-hour basis. Accordingly, the HVAC system is often not operating at its point of maximum design efficiency.